The transforming growth factor-beta (TGF-.beta.) superfamily of cytokines regulate a diverse array of physiologic functions including cell proliferation and growth, cell migration, differentiation, development and apoptosis. This large family includes the TGF-.beta.s, activins, and bone-morphogenic proteins (BMPs) and each subgroup initiates a unique signaling cascade activated by ligand-induced serine/threonine kinase receptor complex formation (Wrana, Miner. Electrolyte Metab., 1998, 24, 120-130). These complexes, once formed, recruit and phosphorylate members of a family of cytosolic proteins, known as Smads. Smads exist as monomers in unstimulated cells but homo- or heterodimerize and translocate to the nucleus activating target gene transcription upon ligand binding. Smads, therefore, connect the pathway of TGF-.beta. signaling from the cell membrane to the nucleus.
To date, nine vertebrate Smads have been identified and these have been divided into subgroups based on their functional role in various pathways. Smad1, 5, and MADH6, which is 80% homologous to Smad1, all mediate signal transduction from BMPs while Smad2 and 3 mediate signal transduction from TGF-.beta.s and activins. Collectively, these Smads are known as the pathway-restricted Smads and can form homo or heterodimers. Smad4 has been shown to be a shared hetero-oligomerization partner to the pathway-restricted Smads and is known as the common mediator. The last two members of the family, Smad6 and 7, act to inhibit the Smad signaling cascades often by forming unproductive dimers with other Smads and are therefore classified as antagonistic Smads (Heldin et al., Nature, 1997, 390, 465-471; Kretzschmar and Massague, Curr. Opin. Genet. Dev., 1998, 8, 103-111).
Smad1 (also known as MADH1, hMAD1, bsp1, dwarfin-A, and JV4-1) is a member of a subgroup of Smad family transcription factors which are regulated by, and act as mediators for, bone morphogenic proteins (BMPs). BMPs play important roles in vertebrate organogenesis and embryogenesis.
Smad1 was first cloned during studies to investigate BMP signaling using expressed sequence tag (EST) sequences having homology to the amino-terminal of the Mad gene of Drosophila (Liu et al., Nature, 1996, 381, 620-623). In these studies, Smad1 was localized to the cytoplasm and nucleus in unstimulated cells but became concentrated in the nucleus in the presence of BMP4 suggesting that Smad1 was a transcriptional activator.
Smad1 has also been shown to mediate other signaling events within the cell. It has been shown to be a target of mitogenic growth factor signaling through the epidermal growth factor (EGF) and hepatocyte growth factor (HGF) receptor protein tyrosine kinases (Kretzschmar et al., Nature, 1997, 389, 618-622).
The role of Smad1 in muscle and bone development has been the most broadly investigated. Smad1 is a signaling molecule for BMP2 and BMP4. In studies of BMP signaling using C2C12 myoblast cells, Smad1 was shown to be involved in convergent signaling pathways that inhibit myogenic differentiation and induce osteoblast differentiation. Overexpression of Smad1 induces the osteoblast phenotype and inhibits muscle specific gene expression in the absence of BMP2 (Yamamoto et al., Biochem. Biophys. Res. Commun., 1997, 238, 574-580).
Currently, there are no known therapeutic agents which effectively inhibit the synthesis of Smad1. Consequently, there remains a long felt need for agents capable of effectively inhibiting Smad1 function. Therefore, antisense oligonucleotides may provide a promising new pharmaceutical tool for the effective and specific modulation of Smad1 expression.